The present invention relates to a fuel injection valve.
Usually changes in the length of a piezoelectric actuator of a fuel injection valve caused by temperature influences are compensated for via hydraulic devices or by selecting suitable combinations of materials.
A fuel injection valve in which the change in length of the actuator is compensated for by an appropriate combination of materials is known from German Patent No. 197 02 066. The fuel injection valve arising from this publication has an actuator that is conducted in the valve housing under spring prestress and that interacts with an actuating part made of an actuating body and a head part, the head part lying on the piezoelectric actuator and the actuating body penetrating an inner recess of the actuator. The actuating body is operably connected to a valve needle. When the actuator is set in motion, the valve needle is actuated against the direction of spraying.
The actuator and the actuating body have at least approximately the same length and are made of a ceramic material or of a material similar to ceramic with respect to its thermal expansion. The result of using materials having the same lengths and thermal expansion coefficients, e.g., INVAR, is that the actuator and the actuating body expand uniformly under the influence of heat and thus do not have an adverse effect on the opening and closing times. An undesired opening of the fuel injection valve between the switching pulses is also avoided.
The disadvantage of this arrangement is above all its limited usability in systems that are subject to large fluctuations in temperature. The arrangement known from German Patent No. 197 02 066 does not achieve the objective due to the nonlinear behavior of the temperature expansion coefficients of piezoelectric ceramics over the temperature curve. As a result, imprecise fuel metering times and amounts occur.
Another disadvantage is the high manufacturing effort required, which is associated with relatively high costs caused in particular by the selection of the materials (e.g., INVAR).
The fuel injection valve of the present invention with the characterizing features of the main claim, on the other hand, has the advantage that the temperature compensation is independent of the thermal expansion coefficient of the piezoelectric ceramic. The thermal expansion is compensated for via damping elements having a speed-dependent transmission behavior for arriving pulses and is thus independent of the selection of the material for the actuating element and valve housing. Thus a secure and precise method of operation of the fuel injection valve is assured.
Advantageous further developments of the fuel injection valve indicated in the main claim are possible by implementing the measures listed in the subclaims.
The simple design of the components from the point of view of manufacturing technology is advantageous. In particular the enclosing and prestressing of the actuator in an actuator housing are advantageous, since the thermal change in length of the actuator does not need to be compensated for by expensive material combinations, but is compensated for by a prestress spring. Thus the entire length of the actuator housing is unaffected by thermal changes in length. Thus by uncoupling the actuator and the valve housing, only a change in position of the actuator housing relative to the valve housing still needs to be compensated for.
The enclosing of a readjusting spring and damping element in a valve shell is also advantageous because of the resulting compact construction.